Piston compressor and method for compressing a cryogenic gaseous medium, in particular hydrogen

ABSTRACT

A piston compressor for compressing a cryogenic fluid medium, in particular in the form of hydrogen is described. It is provided according that an encircling first gap between a first piston and an inner side, facing towards the first piston, of a first cylinder is sealed off by means of at least one seal, which is provided on the first piston, in such a way that leakage medium from the first cylinder interior space passes through the first gap into the interior space of the housing and flows around the rotor and in particular also the stator, wherein the permanent magnets are provided with a coating in order to protect against the medium, in particular in order to protect against hydrogenation in the case of a medium in the form of hydrogen. A method for compressing a cryogenic fluid medium, in particular hydrogen is also disclosed.

The invention pertains to a piston compressor according to claim 1, aswell as to a method for compressing a fluid medium, particularly acryogenic gaseous medium in the form of hydrogen, according to claim 8.

In the context of the present invention, a cryogenic fluid mediumparticularly refers to a fluid medium that has a temperature in therange between 0 K and 130 K. In this case, the fluid medium respectivelyis a gaseous or liquid medium or a mixed phase of a gaseous and a liquidphase. However, the inventive piston compressor can also be operatedwith higher input temperatures, in particular, up to 320 K, i.e. in arange between 0 K and 320 K.

Under standard conditions, gases have a very low density in comparisonwith other energy carriers. In order to efficiently store a gas, it isnecessary to increase the mass of the gas in the available storagespace.

An effective storage of gases is in most cases realized by increasingthe gas pressure. The most popular methods and devices currently usedfor compressing a gaseous medium involve compressor systems such asreciprocating piston compressors, ionic liquid piston compressors,rotary screw compressors or diaphragm-type compressors. Furthermore,known methods for conveying and compressing liquid cryogenic mediums arein most cases realized with piston compressor systems.

Methods and devices of the initially cited type are used, for example,in natural gas and hydrogen compressor stations of the type realized innatural gas fueling stations.

For example, DE-B 102006060147 discloses a fluid processing machine thatis driven by a linear motor, in which, e.g., the stator and the armatureare separated by a can and static seals.

The aforementioned compressors typically operate with gas inputtemperatures that lie in the range of the ambient temperature at theoperating site. In compressor systems that are supplied with liquid gas,it is therefore necessary to transform the liquid into the gaseousstate. The adaptation of the gas input temperature into the compressoris realized with evaporator systems. The energy required for thetemperature increase of the medium in the evaporator systems isobtained, for example, by means of heat extraction from the surroundingsor with an electrical preheating device.

High-capacity cryopumps, in contrast, have to be supplied with a liquidmedium. The liquid tanks required for the supply are extremelyuneconomical due to the limited insulating options and the associatedlosses of liquid hydrogen occurring over long storage times as a resultof unusable boil-off gases.

Due to the temperature differences occurring in compressors, it is aconstructive necessity, for example in piston compressors, to providelength tolerances that are associated with an increase of the clearancevolume. The increased gas re-expansion caused by the relatively largeclearance volume results in a reduced capacity.

In a compressor without clearance volume (e.g. an ionic liquid pistoncompressor)f the crystallization temperature of the ionic fluid limitsthe use at low temperatures. In addition, ionic liquid pistoncompressors require a horizontal installation position in order tomaintain the liquid column.

Furthermore, piston compressors known from the prior art frequentlycannot be constructed in a pressure-encapsulated fashion such that acertain leakage of the medium to be compressed into the surroundings hasto be accepted. Even static seals only make it possible to realize aleakage-free seal under certain conditions. If hydrogen is conveyed andcompressed as it is the case in the system disclosed in DE-B102006060147, a person skilled in the art furthermore faces the problemof hydrogenation of the permanent magnets by the leakage gas.

Based on these circumstances, the objective of the present invention canbe seen in disclosing an improved piston compressor, as well as acorresponding method for compressing a fluid (e.g. gaseous) and, inparticular, cryogenic medium, especially hydrogen.

This objective is attained by means of a piston compressor with thecharacteristics of claim 1. Advantageous embodiments of the inventivepiston compressor are disclosed in the corresponding dependent claimsand described in greater detail below.

According to claim 1, the inventive piston compressor for compressing,in particular, a cryogenic fluid medium, especially in the form ofhydrogen, features a linear motor comprising a stator and an armaturewith permanent magnets, wherein the stator is conventionally designedfor generating a magnetic field in order to move the armature relativeto the stator in a reciprocating fashion along a longitudinal axis,along which the armature extends. The piston compressor also features ahousing of the linear motor that defines an interior, in which thearmature and the stator are arranged, and a first cylinder of the pistoncompressor that is connected to the housing and defines a first cylinderchamber originating at said interior, as well as a first cylinder headof the first cylinder with an inlet, through which the medium can beintroduced into the first cylinder chamber of the first cylinder, andwith an outlet, through which the compressed medium can be dischargedfrom said first cylinder chamber. The piston compressor furthermorefeatures a first piston that protrudes into the first cylinder chamberand extends along the longitudinal axis, wherein this first piston isconnected to the armature such that it is driven by the armature andmoved along the longitudinal axis in a reciprocating fashion, andwherein the first piston is designed for compressing medium located inthe first cylinder chamber when the first piston moves in the directionof the first cylinder head. According to the invention, it is proposedthat an encircling first gap between the first piston and an inner sideof the first cylinder facing the first piston is sealed with at leastone seal provided on the first piston in such a way that only part ofthe medium, which is presently referred to as leakage medium, cantransfer from the first cylinder chamber into the interior of thehousing through this first gap and flow around the armature and, inparticular, the stator, wherein the permanent magnets of the armatureare provided with a coating that serves as protection from said leakagemedium, particularly as protection from hydrogenation and alsoembrittlement when a medium in the form of hydrogen is processed.

The at least one seal provided on the first piston therefore is intendedto seal the first cylinder chamber, but a certain leakage past the sealcan typically not be prevented. This also applies to the second piston(see below).

The inventive piston compressor and the method described below arepreferably realized or designed for compressing a fluid medium. Themedium therefore may be purely gaseous or consist of a mixture of agaseous and a liquid phase. The medium may furthermore also consist of aliquid.

According to a preferred embodiment of the inventive piston compressor,the aforementioned permanent magnets respectively feature aneodymium-iron-boron alloy or are made of this material.

Suitable materials for the inventive permanent magnets generally areferrites and their alloys with the zinc and/or nickel and withmanganese, as well as strontium-ferrites, cobalt-ferrites,barium-ferrites and also alloys of samarium-cobalt andaluminum-nickel-cobalt.

According to another preferred embodiment of the inventive pistoncompressor, it is proposed that the coating of the permanent magnetconsists of a nickel-copper-nickel coating. In other words, thepermanent magnets are initially coated with a layer of nickel, then witha layer of copper and ultimately with another (particularly outermost)layer of nickel. Inventive NiCuNi coatings can be produced, e.g., bymeans of electroplating.

A coating of the permanent magnets for protecting the permanent magnetsmay furthermore also feature one of the following materials or alloys:aluminum oxide, tungsten, molybdenum, gold, platinum, chromium, cadmium,tin, aluminum, silicates of tungsten and molybdenum or nickel-aluminumalloys.

The coating may furthermore also consist of an oxidation layer of thebase material of the permanent magnets that is produced, in particular,prior to the hydrogenation/embrittlement. Such an oxidation layer can beproduced, e.g., by exposing the unprotected permanent magnets to a flowof atmospheric oxygen for a certain time period or preferably by actingupon the permanent magnets with pressure in (particularly high-purity)oxygen.

The overall layer thickness of the respective coating perpendicular toits surface area or perpendicular to the individual layers preferablylies in the range between 3 μm and 500 μm.

It is furthermore preferred that the interior of the housing isfluidically connected to a supply line leading to the inlet on the firstcylinder head by means of a first leakage return line such that theinterior of the housing is acted upon with a pressure corresponding tothe pressure in said supply line, wherein the first leakage return linepreferably branches off a first end section of the interior, and whereinthe first cylinder chamber preferably originates at this first endsection of the interior.

According to another preferred embodiment of the invention, the pistoncompressor features a second cylinder that is connected to the housing.Such a second cylinder makes it possible to carry out a two-stagecompression of the medium to be compressed. The second cylinderpreferably features a second cylinder chamber that originates at theinterior of the housing of the linear motor, as well as a secondcylinder head of the second cylinder, wherein this second cylinder headfeatures an inlet, through which the medium to be compressed can beintroduced into the second cylinder chamber of the second cylinder. Thesecond cylinder head furthermore features an outlet, through which themedium compressed in the second cylinder chamber can be discharged fromthe second cylinder chamber. It is furthermore preferred to provide asecond piston that protrudes into the second cylinder chamber andextends along the aforementioned longitudinal axis. The second pistonpreferably is also connected to the armature such that the second pistonis driven by the armature and moved along the longitudinal axis in areciprocating fashion, wherein the second piston is designed forcompressing medium located in the second cylinder chamber when thesecond piston moves in the direction of the second cylinder head.

An encircling second gap preferably also exists between the secondpiston and an inner side of the second cylinder facing the secondpiston, wherein this second gap is sealed with at least one sealprovided on the second piston in such a way that only part of themedium, which is presently likewise referred to as leakage medium, cantransfer from the second cylinder chamber into the interior of thehousing through this second gap and flow around the armature and, inparticular, the stator (see above).

It is furthermore preferred that the interior on the side of the secondcylinder is fluidically connected to the supply line leading to theinlet on the first cylinder head by means of a second leakage returnline, wherein this second leakage return line particularly branches offa second end section of the interior that lies opposite of the first endsection referred to the longitudinal axis of the piston compressor, andwherein the second cylinder chamber preferably originates at this secondend section of the interior. The two cylinders therefore are arranged toboth sides of the linear motor such that medium is taken in by onecylinder chamber while it is discharged from the other cylinder chamber.

In order to influence the motion of the piston and, in particular, tocontrol the stroke of the first and the second piston, it is preferredto provide a position detection means for detecting the position of thefirst and/or second piston. In this respect, sensorless methodsutilizing construction-related and position-specific ratios of theinductances in the longitudinal and lateral direction referred to thecenter axes of the linear motor may be considered, wherein theposition-specific ratios make it possible to deduce the position. Itwould furthermore be possible to use methods according to EP-B 1746718or WO-A 1992019038.

According to an embodiment of the invention, the position detectionmeans features a displacement transducer that is coupled to the first orthe second piston and generates a first magnetic field (the displacementtransducer may be formed, e.g., by a magnet), as well as a measuringelement that features, e.g., a compression-proof rod, in which amagnetic, elastically deformable body is respectively arranged ormounted. In this case, the displacement transducer is designed forgenerating a longitudinal magnetic field in the measuring element. Theposition detection means is furthermore designed for passing a currentsignal through the measuring element such that a second magnetic fieldis created radially around the measuring element. When the two magneticfields meet, the elastic body is deformed such that a torsional wavepasses through the measuring element and is detected by the positiondetection means. The position of the displacement transducer andtherefore the position of the piston or pistons are deduced based on thetime difference between the current pulse and the arrival of thetorsional wave.

The above-defined objective of the invention is furthermore attained bymeans of a method for compressing a cryogenic fluid medium, especiallyin the term of hydrogen, by utilizing an inventive piston compressor,wherein the fluid medium is compressed at least in the first cylinderchamber by means of the first piston, wherein only part of the medium(referred to as leakage medium) is transferred into the interior of thehousing through this first gap and flows around the armature and, inparticular, the stator, and wherein the permanent magnets areparticularly protected from said medium, especially from hydrogenationand also embrittlement, by the coating of the permanent magnets.

According to an advantageous embodiment of the inventive method, it isfurthermore proposed that medium compressed in the first cylinderchamber is discharged from the first cylinder chamber and compressedonce again in the second cylinder chamber by means of the second piston,wherein only part of the medium, is likewise transferred from the secondcylinder chamber into the interior of the housing of the linear motorthrough the second gap and flows around the armature and, in particular,also the stator therein.

It is furthermore preferred that medium transferred into the interior isreturned to the inlet on the first cylinder head through the firstleakage return line and/or the second leakage return line. The interiorof the housing is therefore acted upon with pressure in theabove-described fashion and allows the return of the leakage medium tothe inlet on the first cylinder head (first compressor stage of thepiston compressor).

As already mentioned above, it is furthermore preferred to detect theposition of the armature, the first piston and/or the second piston,particularly with the above-described position detection means. Thestroke of the first and/or the second piston is preferably controlled insuch a way that the corresponding clearance volume in the first and/orsecond cylinder chamber is reduced in order to increase the efficiencyof the piston compressor. In this context, the respective clearancevolume is the volume defined by the end face of the respective pistontogether with the encircling inner side of the respective cylinder, aswell as the inner side of the respective cylinder head facing thepiston. As the clearance volume diminishes, the piston or its end facecontacts the respective cylinder head.

In the inventive method, it is particularly preferred that the firstmedium is supplied to the piston compressor in liquid form and generallytransferred into the gaseous state shortly before it is introduced intothe first cylinder chamber, wherein ambient heat and/or waste heat ofthe linear motor is preferably used for evaporating the medium.

For compression purposes, it is proposed that the intake temperature ofthe medium to be compressed lies slightly above that of the point ofequilibrium of the corresponding intake pressure. In addition tosupplying a liquid medium to be compressed, it is furthermore alsopossible to supply a cryogenic gaseous medium that is transported intothe first cylinder chamber from a source in a cryogenic gaseous state.

Other characteristics and advantages of the invention are elucidated inthe following description of an exemplary embodiment of the inventionwith reference to the figures.

In these figures:

FIG. 1 shows a partially sectioned view of an inventive pistoncompressor; and

FIG. 2 shows another partially sectioned view of the inventive pistoncompressor according to FIG. 1.

An inventive piston compressor 1 is illustrated in FIG. 1 and FIG. 2.The piston compressor 1 features a linear motor 10 with a stator andwith an armature 20 that can be moved along a longitudinal axis L in areciprocating fashion by means of the stator. In this case, the statorgenerates a magnetic field that cooperates with the permanent magnets Pof the armature 20 such that this armature is moved along thelongitudinal axis L in a reciprocating fashion. In this case, the statorand the armature 20 of the linear motor 10, which is presently realizedin the form of a tubular linear motor 10, are arranged in a housing 11of the linear motor 10 that defines an interior 100 of the linear motor10. Along the longitudinal axis L, the piston compressor 1 features afirst cylinder 30 and a second cylinder 70 to both sides of the housing11, wherein said cylinders respectively enclose a first cylinder chamber300 and a second cylinder chamber 700. These two cylinder chambers 300,700 respectively extend along the longitudinal axis L from a first endsection 100 a and a second end section 100 b of the interior 100 of thehousing 11. The two end sections 100 a, 100 b of the interior 100 of thehousing 11 lie opposite of one another along the longitudinal axis L.

A first piston 31 slides in the first cylinder chamber 300, wherein anencircling gap S is formed between the piston 31 and an inner side 300 aof the first cylinder 30 facing the first piston 31, and wherein saidgap is sealed, in particular, with at least one or preferably severalslotted seals 32 that seal in the dynamic mode, but not statically. Suchslotted seals 32 are particularly characterized by a transection thatmay be produced with a cut extending parallel, oblique orthree-dimensionally offset to the cylinder axis of the seal. The seal 32can be manufactured with such a transection or the transection can beproduced after the manufacture of the seal 32.

A second piston 70 analogously slides in the second cylinder chamber 700and once again contacts an inner side 700 a of the second cylinder 70,in particular, with at least one or preferably several slotted seals 72and thereby seals an encircling second gap S′ between the second piston71 and said inner side 700 a of the cylinder 70.

During the operation or the compressor, the two pistons 31, 71 of thethusly designed compressor stages move along the longitudinal axis L ina reciprocating fashion between their reversal points in the twocylinders 30, 70 and are respectively centered and fixed on the pistonrod and the armature 20 of the tubular linear motor 10 by means of acorresponding device. In this case, a centering adapter 21, 23 isrespectively arranged on the free ends of the armature 20. The centeringadapters 21, 23 are respectively provided with a thread. The counterrings 22, 24 are provided with a corresponding mating thread and screwedto the respectively assigned centering adapter 21, 23, wherein thecentering adapters 21, 23 are on one side screwed to the armature 20 andthe respective piston 31, 71 is clamped between the respective centeringadapter 21, 23 and the respective counter ring 22, 24 such that a rigidconnection between the armature 20 and the pistons 31, 71 results.

In this case, the second piston 71 is realized in two parts and featurestwo sections 710, 720, wherein the first section 710 is fixed on thearmature 20, namely by means of the aforementioned centering adapter 23and the assigned counter ring 24, and wherein the second section 720 ofthe second piston 71 protrudes into the second cylinder 700 from theinterior 100 and compresses medium M taken in by the second cylinderchamber 700 therein.

The end faces of the two cylinders 31, 71 are respectively closed with afirst and a second cylinder head 40, 80, through which the medium hi tobe compressed is introduced into the respective cylinder 30, 70 anddischarged from the respective cylinder in compressed form.

In addition, a centering surface of a flange 12 respectively centers thecylinders 30, 70 relative to the respective flange 12, wherein saidflanges 12 are in turn centered relative to the housing 11 of the linearmotor 10 by means of a centering surface. The end faces of both flanges12 are respectively screwed to the housing 11 and thereby fix thecylinders 30, 70 on the housing 11. The housing 11 and the cylinders 30,70 are sealed relative to the surroundings by means of static seals inthe form of O-rings 101, 102 arranged between the respective flange 12and the housing 11. The housing 11 is thereby pressure-encapsulated. Thelinear motor 10 itself is fixed on its respective base by means of aflange mounting 13 on the housing 11.

In addition, each piston 31, 71 features an annular guide band 33, 73that encircles the respective piston and serves for absorbing radialforces. As already mentioned, parts of the medium M located in therespective cylinder chambers 300, 700 may be transferred into theinterior 100 of the housing 11 through the aforementioned gaps S, S′during the reciprocating motion of the two pistons 31, 71, wherein thisseal leakage is returned to the input side of the piston compressor 1through a first and a second leakage return line 51, 52 in the form of apipeline. In this case, the first leakage return line 51 branches offthe first end section 100 a of the interior 100 and is fluidicallyconnected to a supply line 61, through which medium M to be compressedcan be supplied to an inlet 41 of the first cylinder head 40. This inlet41 can be closed by means of a valve in the form of a suction valve 410.Due to the pressure-encapsulated design and the above-described leakagereturn, the stator and the armature 20 of the linear motor 10 are actedupon with a pressure corresponding to the supply pressure of the pistoncompressor 1 at the inlet 41. Leakage gas M′ accordingly flows aroundthe stator and the armature 20, which in turn transfer heat to theleakage gas M′ during the operation of the compressor. Medium Mcompressed in the first cylinder chamber 300 by means of the firstpiston 31 is discharged through an outlet 42 on the first cylinder head40 that can be closed with a pressure valve 420.

The first cylinder head 40 with the suction and pressure valves 410, 420is positioned on an end of the first cylinder 30 and screwed to thefirst cylinder 30 by means of a coupling ring. The second cylinder head80 is analogously fixed on the opposite end of the second cylinder 70 bymeans of a coupling ring, wherein the second cylinder head 80 alsofeatures an inlet 81 and an outlet 82 that can be respectively closedwith a suction valve 810 and a pressure valve 820. A (not-shown)connecting line leads from the outlet 42 of the first cylinder head 40to the inlet 41 of the second cylinder 80, wherein it is preferred thatthe supply line 61 and said connecting line are respectively realized ina thermally insulated fashion.

During the compression of a hydrogenous medium M, the permanent magnetsP of the linear motor 10 need to be protected from the hydrogenmolecules. The high-performance magnets used in heavy-duty linear motors10 preferably consist of alloys of the elements neodymium-iron-boron.Neodymium is a rear-earth metal. Rare earths are used in metal hydridereservoirs for storing hydrogen. In this case, the effect of adsorptionand subsequent dispersion of the hydrogen atoms in the metal matrix isutilized, but this effect is extremely undesirable in the linear motor10 and would destroy she permanent magnets P over time. The protectionof the permanent magnets P from this hydrogen accumulation is preferablyrealized in the form of a nickel-copper-nickel coating of the permanentmagnets P.

The positioning of the two pistons 31, 71 in the corresponding cylinderchambers 300, 700 is preferably realized with a position detectionsystem 90 that may consist of a suitable displacement transducer systemor of a sensorless control system. In this way, the drive concept bymeans of the tubular linear motor 10 allows highly dynamic interventionsin the motion sequences of the piston compressor 1 during thecompression process. This makes it possible to design the reciprocatingpiston compressor variably and to react to length changes resulting fromthermal expansions by adapting the stroke. Consequently, the adaptationof the piston stroke of both pistons 31, 71 minimizes the clearancevolume and thereby positively affects the capacity of the pistoncompressor 1.

The compression of a cryogenic gaseous medium M, particularly in theform of hydrogen, by means of the inventive piston compressor 1 ispreferably realized in that said hydrogen M is taken in by the firstcompression chamber or the first cylinder chamber 300 from a hydrogenreservoir through the supply line 61 that preferably is thermallyinsulated and through the suction valve 410 of the first cylinder head40, wherein the hydrogen M being taken in is in the next cyclecompressed by the first piston 31 (in that the first piston 31 movestoward the first cylinder head 40) and discharged through the pressurevalve 410 on the first cylinder head 40. The compressed gas M beingdischarged is taken in by the second cylinder chamber 700 through theaforementioned connecting line, which preferably is likewise thermallyinsulated, and through the suction valve 810 of the second cylinder head80 of the second compressor stage, as well as subsequently compressed inthe second cylinder chamber due to a corresponding motion of the secondpiston 71 and discharged through the pressure valve 820. The density ofthe hydrogen has been increased as a result of the pressure increase.Due to the structural shape, the compression by the first stage and theintake by the section stage sake place reciprocally simultaneous.

The inventive compression at low temperature levels likewise results ina lower enthalpy difference of the medium M between the intake state andthe final compressed state than in a process carried out with the samepressure potentials, but at a higher temperature (e.g. gas at roomtemperature). In this way, the effort required for the compression isreduced, which in turn manifests itself in reduced power consumption.

The inventive compression at a low temperature level particularly makesit possible to forgo an intermediate circuit heat exchanger because theintermediate circuit temperature after the first compressor stage stillremains below the typical ambient temperature encountered at theoperating sites due to a temperature increase caused by the compressionprocess.

The inventive compression at a low temperature level is furthermoreassociated with high specific densities of the mediums M such that arelatively high capacity is achieved, particularly for mere gascompressors.

Due to the preferred fully hermetic design of the compressor system 1, adynamic seal of moving parts relative to the surroundings is eliminatedsuch that the known technical advantages of a static seal can beutilized.

The fully hermetic design of the piston compressor system 1 prevents thecontamination of the housing 11 with ambient air. This is realized inthat the housing 11 is constantly acted upon with an overpressure thatcorresponds to the supply pressure of the first compressor stage. Thisallows the return of the gas leakage M′ of the dynamic piston seals 32,72 into the respective intake section or cylinder chamber 300, 700.

The above-described device for respectively fixing the pistons on thelinear motor piston rod and on the so-called armature 20 allows anuncomplicated exchange of both pistons 31, 71 if servicing is required.If the cylinder diameters are adapted simultaneously, the pistondiameters can furthermore be varied in such a way that either higherfinal compression pressures are achieved or the capacity is increased.

In compressor stations 1 that are supplied with a liquid, it is stillpossible to take in the boil-off gas created due to a heat input intothe tank and to utilize this gas for the compression.

Despite the fact that a high process-related capacity of the compressorsystem 1 can be realized, the space requirement remains small incomparison with conventional gas compression systems.

The above-described compressor system 1 can be advantageously operatedhorizontally, as well as vertically.

In an embodiment of the piston compressor 1 for compressing hydrogen, itis proposed that the first piston 31 has a diameter of 42 mm and thesecond piston 71 has a diameter of 16 mm. The frequency of the pistonmotion preferably lies at 10 Hz, the mass flow of the medium Mpreferably amounts to 10 kg/h and the oscillating inertia forces(composed of the individual oscillating inertia forces of theoscillating components: armature 20, centering adapter 21, 23, counterring 22, 24, piston 31, 37, seal 32, 72 and guide band. 33, 73)preferably amounts to 50 kg. The stroke of the pistons 31, 71 preferablyamounts to 120 mm. The piston motion preferably has a harmonic function.The resulting compression force amounts to 10 kN and the footprint ofthe compressor 1, i.e. the surface projected in a top view, measuresapproximately 2.5 m×1 m. The attainable maximum force of the linearmotor 10 lies at 13.8 kN and the attainable maximum speed of the linearmotor 10 lies at 4.1 m/s. In this case, the rated power of the linearmotor 10 amounts to 26.6 kW.

The hydrogen is preferably introduced into the first cylinder chamberwith a temperature of 60 K and a pressure of 6 bara, compressed anddischarged with a temperature of 184K and a pressure 133 bara, whereinthe hydrogen is then pressed into the second cylinder chamber 100, inwhich it is compressed once again, and ultimately discharged with atemperature of 288 K and a pressure of 600 bara.

LIST OF REFERENCE NUMERALS

-   1 Piston compressor-   10 Linear motor-   11 Housing-   12 Flange-   13 Flange-   20 Armature-   21 Centering adapter-   22 Counter ring-   23 Centering adapter-   24 Counter ring-   30 First cylinder-   31 First piston-   32 Seal-   33 Guide band-   40 First cylinder head-   41 Inlet-   42 Outlet-   51 First leakage return line-   52 Second leakage return line-   61 Supply line-   70 Second cylinder-   71 Second piston-   72 Seal-   73 Guide band-   80 Second cylinder head-   81 Inlet-   82 Outlet-   90 Position detection means-   91 Displacement transducer-   92 Measuring element-   100 Interior-   100 a First end section-   100 b Second end section-   101, 102 O-rings (static seals)-   300 First cylinder chamber-   300 a Inner side-   400 Housing-   410 valve (suction valve)-   420 Valve (pressure valve)-   700 Second cylinder chamber-   700 a Inner side-   810 Valve (suction valve)-   820 valve (pressure valve)-   M Medium (e.g. hydrogen)-   M′ Leakage medium-   P Permanent magnets (armature)-   B Coating-   L Longitudinal axis-   S First gap-   S′ Second gap

1. A piston compressor for compressing a cryogenic fluid mediumcomprising: a linear motor that comprises a stator and an armature withpermanent magnets, wherein the stator is designed for driving thearmature by generating a magnetic field in order to move the armaturerelative to the stator in a reciprocating fashion along a longitudinalaxis, along which the armature extends, a housing of the linear motorthat defines an interior, in which the armature and the stator arearranged, a first cylinder that is connected to the housing and definesa first cylinder chamber that originates at said interior, a firstcylinder head of the first cylinder with an inlet, through which themedium can be introduced into the first cylinder chamber, and with anoutlet, through which the compressed medium can be discharged from thisfirst cylinder chamber, a first piston that protrudes into the firstcylinder chamber and extends along the longitudinal axis, wherein thisfirst piston is coupled to the armature such that the first piston isdriven by the armature and moved in a reciprocating fashion along thelongitudinal axis, wherein the first piston is designed for compressingmedium located in the first cylinder chamber during a motion of thefirst piston toward the first cylinder head, characterized in that anencircling first gap between the first piston and an inner side of thefirst cylinder facing the first piston is sealed with at least one sealprovided on the first piston in such a way that medium is transferredfrom the first cylinder chamber into the interior of the housing throughthis first gap and flows around the armature, wherein the permanentmagnets are provided with a coating as protection from this medium. 2.The piston compressor according to claim 1, characterized in that thepermanent magnets feature an alloy comprising neodymium, iron and boronwith the composition Nd₂Fe₁₄B.
 3. The piston compressor according toclaim 1, characterized in that the coating is selected from the groupconsisting of the following coatings: a nickel-copper-nickel coating,wherein the coating is produced by initially applying at least one layerof nickel, then a layer of copper and ultimately another layer ofnickel, and wherein the overall layer thickness of the coating lies inthe range between 3 μm and 500 μm, a coating that is selected from thegroup consisting of aluminum oxide, tungsten, molybdenum, gold,platinum, chromium, cadmium, tin, aluminum, silicates of tungsten andmolybdenum or nickel-aluminum alloys, and a coating that comprises atleast one oxide of the permanent magnet material, wherein this coatingis produced by bringing the permanent magnets in contact with oxygen. 4.The piston compressor according to claim 1, characterized in that theinterior is fluidically connected to a supply line leading to the inleton the first cylinder head by means of a first leakage return line suchthat the interior is acted upon with a pressure corresponding to thepressure in said supply line, wherein this first leakage return linebranches off a first end section of the interior, and wherein the firstcylinder chamber originates at this first end section of the interior.5. The piston compressor according to claim 1, characterized in that thepiston compressor furthermore comprises: a second cylinder that isconnected to the housing and defines a second cylinder chamber thatoriginates at the interior, as well as a second cylinder head of thesecond cylinder, wherein the second cylinder head has an inlet, throughwhich the medium can be introduced into the second cylinder chamber, andan outlet, through which the compressed medium can be discharged fromthis second cylinder chamber, and a second piston that protrudes intothe second cylinder chamber and extends along the longitudinal axis,wherein this second piston is coupled to the armature such that thesecond piston is driven by the armature and moved in a reciprocatingfashion along the longitudinal axis, wherein the second piston isdesigned for compressing medium located in the second cylinder chamberduring a motion of the second piston toward the second cylinder head,and wherein an encircling second gap between the second piston and aninner side of the second cylinder facing the second piston is sealedwith the least one seal provided on the second piston in such a way thatmedium is transferred from the second cylinder chamber into the interiorof the housing through this second gap and flows around the armature. 6.The piston compressor according to claim 4, characterized in that theinterior is fluidically connected to the supply line leading to theinlet of the first cylinder head by means of a second leakage returnline, wherein this second leakage return line branches off a second endsection of the interior, and wherein the second cylinder chamberoriginates at this second end section of the interior.
 7. The pistoncompressor according to claim 1, characterized in that a positiondetection means is provided for detecting the position of the firstand/or the second piston, wherein said position detection meanscomprises a displacement transducer that is coupled to the first or thesecond piston and designed for generating a first magnetic field, aswell as for being moved along a measuring element, which extends in theinterior along the longitudinal axis and comprises a magnetic,elastically deformable body, during each reciprocating motion of thearmature, wherein the position detection means is designed forgenerating a second magnetic field around the measuring element byapplying a current signal to the second measuring element such that atorsional wave is generated in the elastically deformable body due tothe interaction of the two magnetic fields, and wherein the positiondetection means is furthermore designed for detecting said torsionalwave and for determining said position based on the time differencebetween the application of the current signal and the detection of thetorsional wave.
 8. A method for compressing a cryogenic fluid medium byutilizing a piston compressor comprising: a linear motor, that comprisesa stator and an armature with permanent magnet, wherein the stator isdesigned for driving for driving the armature by generating a magneticfield in order to move the armature relative to the stator in areciprocating fashion along a longitudinal axis, along which thearmature extends, a housing of the linear motor that defines aninterior, in which the armature and the stator are arranged, a firstcylinder that is connected to the housing and defines a first cylinderchamber that originates at said interior, a first cylinder head of thefirst cylinder with an inlet through which the medium can be introducedinfo the first cylinder chamber, and with an outlet, through which thecompressed medium can be discharged from this first cylinder chamber, afirst piston that protrudes into the first cylinder chamber and extendsalong the longitudinal axis, wherein this first piston is coupled to thearmature such that the first piston is driven by the armature and movedin a reciprocating fashion along the longitudinal axis, wherein thefirst piston is designed for compressing medium located in the firstcylinder chamber during a motion of the first piston toward the firstcylinder head, characterized in that an encircling first gap between thefirst piston and an inner side of the first cylinder facing the firstpiston is sealed with at least one seal provided on the first piston insuch a way that medium is transferred from the first cylinder chamberinto the interior of the housing through this first gap and flows aroundthe armature, wherein the permanent magnets are provided with a coatingas protection from this medium, wherein the medium is compressed atleast in the first cylinder chamber by means of the first piston,wherein part of the medium is transferred into the interior of thehousing through the first gap and flows around the armature, and whereinthe permanent magnets are protected from said medium.
 9. The methodaccording to claim 8, characterized in that medium compressed in thefirst cylinder chamber is discharged from the first cylinder chamber andcompressed once again in the second cylinder chamber by means of thesecond piston, wherein part of the medium transferred from the secondcylinder chamber into the interior of the housing through the second gapand flows around the armature.
 10. The method according to claim 8,characterized in that medium transferred into the interior is returnedto the inlet on the first cylinder head through the first leakage returnline and/or the second leakage return line.
 11. The method according toclaim 8, characterized in that the position of the armature, the firstpiston and/or the second piston is detected, and that the stroke of thefirst and/or the second piston is controlled in such a way that theclearance volume in the first and/or the second cylinder chamber isreduced.
 12. The method according to claim 8, characterized in that themedium is supplied to the piston compressor in liquid form andtransferred into the gaseous state before it is introduced into thefirst cylinder chamber wherein ambient heat and/or waste heat of thelinear motor is used for evaporating the medium.
 13. The pistoncompressor according to claim 1, characterized in that the cryogenicfluid medium is in the form of hydrogen.
 14. The piston compressoraccording to claim 1, characterized in that the protection is againsthydrogenation when a hydrogen medium is being processed.
 15. The methodaccording to claim 8, characterized in that the cryogenic fluid mediumis in the form of hydrogen.
 16. The method according to claim 8,characterized in that the protection is against hydrogenation when ahydrogen medium is being processed.